Typescript error: Property 'flat' does not exist on type '[string, unknown][]'

Question

I need to use .flat() on an array, but Typescript claims:

Property 'flat' does not exist on type '[string, unknown][]'

I've added "es2019 to my tsconfig file:

    "lib": ["es2015", "es2019", "dom"],

The function where flat() is used is:

const legendSeries = (series) => {
    const formattedSeries = {};
    series.map((serie) =>
      Object.entries(serie).map((s) => {
        if (s[0] in formattedSeries) {
          return (formattedSeries[s[0]].count += s[1]);
        }
        return (formattedSeries[s[0]] = { id: s[0], label: s[0], count: s[1] });
      })
    );
    return Object.entries(formattedSeries)
      .flat()
      .filter((_, i) => i % 2 !== 0);
  };

The error persists. How to fix this (it's a React project in case it might be helpful)?

Answer 1

I think this issue is because typescript doesn't have access to array.flat(), in such a case it will throw an error of not have knowledge of flat(), as mentioned above by @codejockie. You need to update your config file like { "compilerOptions": { "target": "es5", "lib": [ "es2019" ] } }

we need to add es2019 or es2019.array to --lib setting for TypeScript so that it starts recognizing array.flat() and flatMap().

After making the changes. Please restart your project if the changes don't reflect.